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Dr. Boost: Piston-type bypass valves (long).....
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Posted by zeke (more from zeke) on Tue, 23 Apr 2002 12:11:15 Share Post by Email
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I've been trying to locate a pressure leak in my Saab. I thought it
was possibly due to a crack in the plastic end-tank of my intercooler,
but upon removal and inspection, there appears to be no crack. This
isn't a big thing because I am replacing the IC with a new all aluminum
one (KylarJonsson- 50mm thick). However, after reading Alains reply to
my thread below, I'm starting to think that maybe my piston-type BPV may
be responsible. I'll use Alains advise and plug the end of the rubber
hose where it attaches to the compressor outlet and apply pressure at
the other end, where it attaches to the throttle body. I've visually
inspected the four rubber hoses that join all the aluminum piping in
this pressurized circuit and have not found any holes or cracks. I
should add, my engine and all the ancillaries, except for the intercooler
are 4 years old with about 45k miles. Now, my question is this: on piston
type BPV's is the pressure adjustment (adding washers) a linear response?
The BPV's response is effected by differential pressures across the
throttle body housing. Maybe the minimal spring tension is not enough
to keep the piston/valve closed at higher boost. If I'm reasoning this
properly, then as the pressure and flow increase at higher rpms, so too
does the pressure drop through the intake piping and throttle body housing.
If this pressure drop is greater than the spring tension holding the valve closed then you start to leak boost pressure. IIRC, a pressure differential of about 4.5 psi. will cause the valve to leak out precious
boost! However, adding more washers to keep the valve closed means that
the valve may not be open as much at idle as before. Adding more
washers will increase the boost pressure due to an increase in spring
tension, but the pressure differential/drop through the intake piping
and throttle body will also go up. I would think that the increase in
spring tension by adding washers gives a linear response. However, the
boost pressure increase will not be linear because the increased
pressure loss may not be linear with flow & pressure. What I'm getting
at is maybe I didn't add enough washers to the piston? And I wonder if
having the valve not as open at idle will affect the response of
spool-up?



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